(a) Field of the Invention
The present invention relates to liquid crystal displays (referred to as an LCD hereinafter) and manufacturing methods thereof, and more particularly, to electrostatic discharge prevention circuits for LCDs and manufacturing methods thereof.
(b) Description of the Related Art
A liquid crystal display (LCD), which is one type of flat panel display (FPD), includes two substrates having transparent electrodes and a liquid crystal layer interposed between the substrates. In the LCD, light transmittance is controlled by varying the voltages applied to the liquid crystal layer.
On a thin film transistor (TFT) substrate of the LCD, N gate lines and M data lines, cross one another and define a plurality of pixels in an Nxc3x97M matrix. A pixel electrode is formed for each pixel, and is coupled to the gate and the data lines by a switching device such as the TFT. The TFT controls display signals transmitted through the data line according to the states of the scanning signals transmitted through the gate line.
The majority of the LCD manufacturing process is performed on a glass substrate. Since the glass substrate is nonconductive, electric charges generated suddenly cannot be dispersed. Such an electrostatic charge may damage the insulating films or TFTs.
In the LCD manufacturing process, the high voltage electrostatic charges generated after the assembly of the TFT substrate and a color filter substrate may decrease the substrate quality, even though the charge amount is small. Also, the electrostatic charge generated during the cutting step of the substrate flows into the active area of the pixel regions through gate and data pads and damages the channels of the TFTs near the pads.
FIG. 1 shows a layout view of the conventional LCD substrate which is damaged by an electrostatic discharge. As shown in the drawing, the LCD panel includes a TFT substrate 10 and a color filter substrate 20. A pad area 30, in which pads are formed to connect each wire of the TFT substrate 10 to driving circuits, and an active area 40, where actual images are displayed, are separately formed on the TFT substrate 10.
Lines 50 in the active area 40 illustrate pixels of TFTs damaged by electrostatic charges. If electrostatic charges are generated in the pad area 30 and propagate into the active area 40, the channels of the TFTs next to the pads are damaged and the channel quality is deteriorated.
The deteriorated TFT is shown in FIG. 2. As shown in the drawing, a gate line 60 and a data line 80 cross each other. An edge of a gate electrode 61, extended from the gate line 60, overlaps an end of a source electrode 81 which is extended from the data line 80. An edge of the gate electrode 61, opposite the edge overlapping with the source electrode 81, overlaps with a drain electrode 82. A semiconductor film 70 is formed on the overlapping portion of the gate electrode 61, source electrode 81 and drain electrode 82.
The electrostatic charges entering into the TFT will be discharged with sparks between the source electrode 81 and the drain electrode 82, thereby damaging the semiconductor film 70.
To reduce the damage caused by electrostatic discharges, a shorting bar is typically used to disperse the electrostatic charges. The shorting bar is located at the edge of the substrate and connects all the metal wires. However, the shorting bar alone cannot prevent damage caused by a large electrostatic discharge. Moreover, the electrostatic charges cannot be prevented from entering into the substrate after the shorting bar is removed.
In manufacturing the LCD panel, a polarizer is attached after performing a visual display test by applying signals to the shorting bar. Then, the mother substrate is cut into individual LCD substrates, liquid crystal is injected between the substrates, and the injection holes are sealed. The shorting bar is removed as the substrate is cut. In another visual display test, different test signals are applied to adjacent data lines via corresponding parts by probes and driving circuits are attached to the LCD panel.
As mentioned above, since the shorting bar is removed as the substrate is cut, it is difficult to protect the substrate against the electrostatic charges subsequent to the removal of the shorting bar. Moreover, since the polarizers are attached after the simple test, in which only one signal is applied to every wire, by using the shorting bar, the polarizers will be evenly attached to the damaged LCD panel. If a panel is determined to be damaged in a subsequent test, it has to be discarded along with the polarizers, thereby increasing overall manufacturing costs of the LCD.
It is an object of the present invention to provide a liquid crystal display having a substrate that is safe from electrostatic charges, regardless of the strength thereof.
It is another object of the present invention to provide a liquid crystal display that prevents electrostatic charges from entering into the substrate after a shorting bar is removed, thereby minimizing pixel defects.
It is yet another object of the present invention to provide a method for manufacturing a liquid crystal display that prevents an LCD panel from being damaged by an electrostatic discharge, while reducing the manufacturing cost of the LCD.
The present invention provides a LCD with a plurality of spark inducing circuits. The spark inducing circuits dissipate electrostatic charges generated in wires of a TFT substrate. A plurality of electrostatic charging circuits that dissipate the electrostatic charges by storing them.
The spark inducing circuit includes a plurality of the TFTs, connected in series between two adjacent wires and gate electrodes of which are connected to that of the other; and two capacitors, one electrode of which is connected to the gate electrode of the TFTs and the other electrode of which is connected to the adjacent wire. Since a plurality of the spark inducing circuits are connected in parallel between the adjacent wires, if electrostatic charges generate in the wires, sparks occur in the TFTs of the spark inducing circuits, which induces a strong current between the source and the drain electrode of the TFTs. This surge current transforms into joule heat, thereby losing its strength. Therefore, the TFTs in an active area are protected from the electrostatic discharge. The electrostatic charges generated in a wire also disperses to the adjacent wires. The spark inducing circuit formed by connecting a TFT and a capacitor in series between each wire and a common electrode.
In another aspect, the spark inducing circuit may be formed with a TFT, whose gate electrode and drain electrode are respectively connected to the same wire, and source electrode is connected to a dummy line; and a capacitor formed between the wire and the drain electrode.
A circuit for dispersing electrostatic charges comprises a resistor and a capacitor connected in series between a data line and a dummy gate line and another resistor connected between an adjacent data line and the capacitor to replace the spark inducing circuit.
The electrostatic charging circuit comprises a first electrostatic charging circuit, which is formed outside a sealing material that assembles the TFT substrate and the corresponding substrate and a second electrostatic charging circuit, which is formed inside a sealing material. The first electrostatic charging circuit has two capacitors connected in series to each other between two adjacent wires. A number of the first electrostatic charging circuits may be connected in parallel to the adjacent wires. The second electrostatic charging circuit, which prevents electrostatic charges from entering the active area, comprises capacitors formed between each wire and a common electrode. The capacitor includes wires and an additional corresponding electrode coupling to the common electrode. The corresponding electrode over the gate line is made of a metal used for forming the data line, and the corresponding electrode over the data line is made of a metal used for forming the gate line. The first and the second electrostatic charging circuits remove the electrostatic charges generated in the wires by storing them.
To protect the TFT substrate from electrostatic charges, a shorting bar, which links all the wires formed on the TFT substrate, is formed inside a cutting line of the substrate. Since the shorting bar remains on the substrate even after the TFT substrate is divided into a plurality LCD panels, it is still possible for the TFT substrate to be protected by the shorting bar.
To protect the LCD from electrostatic charges occurring in the manufacturing process, an electrostatic discharge protection circuit, a TFT and wires are formed on a substrate. A shorting bar is formed inside the cutting line of the substrate, and the substrate is cut into several TFT substrates. Next, individual LCD panels are assembled and the shorting bar is removed by edge-grinding. After visual display tests by applying test signals to each of the wires, polarizers are attached on the LCD panel of no defect. Driving circuits are then connected to the LCD panels.
In the manufacturing method of the current invention, the LCD panel is protected against electrostatic charges since the cutting step, injection of the liquid crystal and the sealing of an injection hole are all performed while the shorting bar remains on the LCD panel. Moreover, it is possible to reduce the manufacturing cost since polarizers are attached on the good LCD panels only.
In another embodiment of the present invention, dummy lines are laid outside an active area that forms a plurality of dummy pixels by intersecting gate lines and data lines. A dummy TFT connecting the dummy line is formed for each dummy pixel.
In the above, the width to length ratio of the dummy TFT channel is higher than that of the TFT channel formed in the active area, or more than one dummy TFTs are formed for one dummy pixel. Accordingly, electrostatic charges are dispersed through the dummy TFT when generated.
A dummy pixel electrode, which is connected to the TFT, is formed in the dummy pixel, and a black matrix to cover the dummy pixel is formed on one of the two substrates.
Generally, electrostatic charges that generate at the beginning or at the end of each step pass through dummy gate lines and dummy data lines of the dummy pixels surrounding the active area and deteriorate the dummy TFT first. Therefore, the TFTs of the active area connected to the gate and the data line, are protected against electrostatic charges. Here, damaged dummy pixels do not affect the LCD quality.
The shape of the dummy TFT may vary to effectively induce electrostatic charges. It is preferable that the width to length ratio of the dummy TFT channel is higher than that of the TFT in the active area. A plurality of dummy TFTs may be formed in the dummy pixel.
Meanwhile, an electrostatic charge dispersing pattern, consisting of two electrodes and a semiconductor pattern, is formed outside an active area to dissipate electrostatic charges through the channel of the semiconductor pattern. To effectively discharge electrostatic charges, the ends of the electrodes may be pointedly formed and a capacitor is formed at the end of the semiconductor pattern. A plurality of the discharge patterns may be connected in parallel to one or two wires.